This invention relates to the field of wireless communications, and, more specifically, to wireless receivers that use automatic frequency correction in demodulation.
Signals in wireless communications systems between mobile stations and base stations are subject to various conditions that can degrade the signal. For example, a receiver usually receives a signal from multiple directions (e.g., directly from the transmitter, and/or reflected off of many different topographical and man-made objects). The various signals are potentially out of phase with each other and thereby cancel each other out to some degree, reducing signal strength (known in the art as xe2x80x9cfadingxe2x80x9d). Such signal fading generally occurs spatially over the area of the system with specific areas potentially having significant fading, which can cause complete signal loss.
Further, signals are subject to the Doppler effect as mobile stations move about the wireless communications system. As is known in the art, whenever relative motion exists between the signal source/transmitter and signal receiver, there is a Doppler shift of the frequency components of the received signal. When a mobile station moves toward a base station, there is a positive Doppler shift (i.e., the frequency of the signal increases). When a mobile station moves away from a base station, there is a negative shift (i.e., the frequency of the signal decreases). The shift in frequency results in the maximum signal strength being at the shifted frequency rather than the assigned frequency, with the signal strength being significantly less at the assigned frequency (as perceived by the receiver). If the mobile station also happens to pass through an area in the system that is subject to fading, a significant loss in signal strength can result. The net result of these and other factors is that the transmitted signal is distorted by the time it reaches the receiver. In a mobile station, this can result in distortion objectional to the ear or even loss of the signal.
In order to account for this distortion, channel estimates are used to determine the amplitude and phase distortion at known pilot symbols in the signal. Correction factors for the other symbols in the signal are interpolated from the channel estimates. As an example, signals are transmitted in the IS-136 system with 162 symbols, each symbol comprising two bits. In a proposed extension of the IS-136 system, the 162 symbols have at predetermined known locations Pi in the signal predetermined, known pilot symbols Spi(where i=1 to n, n being the number of pilot symbols used).
Correction factors (i.e., channel estimates) interpolated from the channel estimates at the pilot symbols can be used to estimate the most likely value for each data symbol. That is, channel estimates derived from the pilot symbols are interpolated to determine the channel estimates for demodulating the other symbols. Such known interpolators try to fit a certain characteristic to the channel estimates obtained at the pilot symbols.
Typically, the interpolator uses knowledge of the statistical variations of the channel (fading) in time, and knowledge of the maximum Doppler frequency expected to be encountered, which depends on the maximum expected speed of the mobile station and the carrier frequency of operation.
Another problem that typically arises in wireless communications is one of frequency offset. This occurs because the frequency generated by the local frequency reference at the mobile station is different from that used by the base station in its transmission. Such frequency offset needs to be corrected to permit reliable demodulation of the data symbols.
In a communication system such as IS-136, automatic frequency correction can be handled in the process of demodulation, as described in U.S. Pat. No. 5,093,848 awarded to A. K. Raith of Ericsson. In this patent, an error signal from the demodulator is passed through a loop that produces at its output a smoothed estimate of the frequency offset. This frequency estimate is used to correct for the frequency error that is encountered.
In a system that employs pilot symbols and interpolation, the above method is not directly applicable, because the knowledge of the fading channel statistics and maximum Doppler frequency used by the interpolator are invalid in the presence of frequency offset. Thus, the interpolated channel estimates are erroneous, and this leads to degraded performance of the demodulator. In such systems, it is necessary to perform automatic frequency correction before interpolation can be done to produce interpolated channel estimates.
A method to perform the above has been disclosed in the paper xe2x80x9cFrequency Offset Compensation of Pilot Symbol Assisted Modulation in Frequency Flat Fadingxe2x80x9d by Wen-Yi Kuo and Michael P. Fitz, which appears in the IEEE Transactions on Communications, Nov. 1997, pp. 1412-16, and is incorporated herein by reference. The method used in this paper finds channel estimates at the locations of the pilot symbols and tries to fit a sinusoid to these channel estimates, along with the knowledge that fading has occurred with a certain statistical variation. The method deduces the frequency offset from the best fit sinusoid. The main shortcoming of the above solution is that it takes a long time to converge.
The present invention is directed toward overcoming one or more of the problems set forth above.
According to one aspect of the present invention, a method is provided for selecting a frequency offset for automatic frequency correction in a pilot symbol assisted demodulator. The method comprises the steps of receiving a signal comprising a plurality of symbols having discrete possible values, and pilot symbols at predetermined locations in the signal, each pilot symbol having a predetermined value, selecting a set of postulates defining a range of frequency offsets, multiplying the pilot symbols and the selected ones of the plurality of the symbols by a sinusoid of each one of the set of postulates to compensate the waveforms for the postulated errors, and generating an estimated channel for each set of error-compensated pilot symbols. The estimated channel of each one of the set of error-compensated pilot symbols is then interpolated and metrics are generated for each one of the sets of error-compensated selected symbols using the symbols"" discrete possible values and the interpolated estimated channel. The postulate with the minimum accumulated metric is used in demodulating the symbols.
According to a further aspect of the invention, the signal is time-aligned prior to selecting the postulates. According to another aspect of this invention, the postulates comprise wi and the step of multiplying the signal by a sinusoid of the postulate comprises multiplying the signal by exe2x88x92j2nwin. Further, the defined set of received symbols comprises Rn, the interpolate estimated channel comprises Cn and the discrete possible data symbol values comprise Sj wherein the step of generating the metric comprise finding a minimum for |Rnxe2x88x92Cn*Sj|2.
In accordance with another aspect of this invention, the selected ones of the plurality of symbols comprises symbols at one or more vulnerable locations in the signal. A vulnerable location may be defined to be the mid-point between two pilot symbols. According to another aspect of this invention, coarse frequency correction of the signal is performed before the step of selecting postulates.
According to another aspect of this invention, an automatic frequency correction device is described in a pilot symbol assisted demodulator. The device receives a plurality of symbols, wherein ones of the symbols are predefined pilot symbols, and delivers a frequency offset for use in demodulation of the signal. The device includes an error compensator for multiplying the pilot symbols and selected ones of the symbols by sinusoid of each of a set of frequency offset postulates, a channel estimator configured to estimate a channel for each of the set of error-compensated pilot symbols, and an interpolator configured to interpolate the estimated channels. The automatic frequency correction device further includes a demodulator that generates a metric for each of the error-compensated symbols using the estimated channels and a selector for selecting one of the set of postulates with the minimum accumulated metric for use as a frequency offset in the automatic frequency compensation.
In accordance with a further aspect of this invention, the automatic frequency correction device further includes means for time aligning the symbols prior to error compensation. In accordance with a still further aspect of the invention, the automatic frequency correction device further includes means for coarse frequency correction of the symbols prior to error compensation. In accordance with a further aspect of this invention, a best postulate of the frequency offset is determined. After correction by this postulate, the demodulation is performed and the error signal in the demodulation process is passed through a frequency loop that produces, at its output, a smoothed estimate of the residual frequency offset that is not accounted for by the postulate produced earlier.